The invention relates to a method for identifying the type of occupancy of a supporting surface, such as a motor vehicle seat, using force sensor-assisted signals.
The invention further relates to a device for identifying the type of occupancy of a supporting surface, particularly a motor vehicle seat. In this particular device, at least three force sensors or at least one axis of rotation in conjunction with at least one force sensor are arranged on the supporting surface. The sensors are used to measure the forces acting on the supporting surface by means of measurement electronics connected to the sensors. A control unit connected to the measurement electronics enables the control of a device as a function of the measured values.
The specific detection and distinction between loads on supporting surfaces, e.g., seats, beds or loading areas, is a development goal of modern weighing technology, which has meanwhile become increasingly important beyond this field, e.g., in automotive engineering.
In motor vehicles, for reasons of safety and to prevent injuries, an airbag, in the event of a collision, should not be activated, or should be less forcefully activated or inflated if the corresponding vehicle seat is occupied by a child in a child seat.
It is known to arrange force sensors in the seat, which measure the weight and infer the type of occupancy from the total weight measured. The measurement result can be used by an airbag control unit.
The problem with this arrangement is that the usual securing of the child seat by means of the safety belt can mislead the control unit. When the child seat is secured by the safety belt, the belt generates increased forces as a result of tightening which signal greater weight forces to the built-in sensors than would correspond to the actual weight of the child and the child seat. As a result, the airbag control can incorrectly infer that a light adult instead of a child is occupying the seat. In the event of a collision, the airbag would then be deployed normally although a child seat and a child occupy the seat. Belt forces of up to 100 kg are possible when a child seat is secured, so that a distinction between an adult and a child seat, which is based only on the measured mass occupying the seat, becomes completely impossible. Particularly when the tightening of the belt is combined with an electric height adjustment of the vehicle seat after the child seat has been secured, high belt forces of up to 100 kg are not unusual. There are also child seat models on the market, which due to their own substantial weight combined with that of a child can easily fall within the weight range of what is known in automotive engineering as the “5% woman,” even if the belt is not, or is only slightly, tightened, so that a distinction based on the mass occupying the seat can no longer be made.
An improved method for determining the actual weight occupying a motor vehicles seat is disclosed in WO 01/18507 A1 (Method and apparatus for measuring seat occupant weight). Here, the center of gravity or the distribution of mass on the seat is used in addition to the total weight. For this purpose, four force sensors are integrated in the seat, one sensor in each corner area of the seat. The method is based on the fact that an increased tightening of the seat belt when the child seat is secured, in contrast to the occupancy and securing of the belt by an adult, results in an asymmetry of the weight distribution in the area of the sensor adjacent to the belt mounting. An increased force vector is directed at the belt mounting. If a measured asymmetry relative to the total weight exceeds a predefined limit, a compensation factor is calculated and used to downwardly adjust the measured total weight. This is to prevent maloperation of an airbag system. In a preferred embodiment, fixed limits (minimum weight/maximum weight) apply to the correction factor. Outside these limits the correction factor is not used, and within them a linear dependence is used.
A disadvantage of the known method is that weight distributions occur in certain child seat models, which do not produce the asymmetry or produce less asymmetry than required for the functioning of the method, so that a misinterpretation of the seat occupancy cannot be excluded. Although shifts in the center of gravity when child seats are secured can occur, they depend to a large degree on the design of the corresponding child seat and the handling by the corresponding operator. Moreover, it cannot be excluded that an adult, by assuming a sitting position with unilateral loading of the seat, causes an asymmetric weight distribution. Thus the prior-art method cannot satisfy the particularly high reliability requirements of systems for safety-related vehicle equipment.
WO 01/12473 A1 discloses a vehicle occupant position detector and an airbag control system. Here, in addition to the total weight of the seat occupant and a shift in the center of gravity outside a certain range, the inclination angle of the seat back is taken into account. The purpose is to activate an airbag or deactivate or activate it with less force as a function of the occupant weight and the sitting position. The inclination angle of the seat back is determined indirectly by a mass ratio of a front seat area to a rear seat area measured by two force sensors each and by comparing the measured values with predefined stored comparison values. For an erect sitting position or a sitting position in the front area of the seat, a higher value of the weight ratio is expected than for a reclining sitting position of an occupant. If a rather erect or forwardly shifted sitting position is deduced, the airbag is activated with less force or is deactivated. In addition, a distinction is drawn between occupancy by an adult and occupancy by a child on the basis of the total occupant weight measured. If the measured value falls below a certain minimum, the airbag is deactivated or deployed with less force.
The drawback, as explained above, is that high weight forces can occur even if the occupant is a child in a child seat. This can occur if the child seat is a particularly heavy model or due to the tightening of the belt. As a result, using the total weight as the only criterion to distinguish between a seat occupied by a child seat and a seat occupied by an adult is not practical. A further drawback is that child seats, due to different designs, sizes and weights and due to the tightening of the belt when they are secured, can produce different weight distributions resulting in a center of gravity in a front or a rear seat area, or they can produce a uniform weight distribution. For an adult, the weight ratio according the known method could be used to determine whether the occupant is in a rather erect or reclining sitting position, but because the identification of the adult is relatively uncertain based only on mass, a child seat with a rearward shift of the center of gravity could be misidentified as a reclining person. This would erroneously lead to activation of the airbag, even though the seat is occupied by a child in a child seat.
In summary, a measurement of the weight and the weight distribution does not readily provide a reliable distinction between a seat occupied by a child in a child seat and a seat occupied by an adult.